1. Field of the Invention
The invention in general relates to frequency standards, and more particularly to an atomic clock of the type which utilizes an optically pumped cell containing a vapor.
2. Description of Related Art
Atomic clocks are utilized in various systems which require extremely accurate and stable frequencies, such as in a GPS (global positioning system) and other navigation and positioning systems, as well as in cellular phone systems and scientific experiments, by way of example.
In one type of atomic clock, a cell containing an active medium such as cesium (or rubidium) vapor is irradiated with both optical and microwave power whereby light from an optical source pumps the atoms of the vapor from a ground state to a higher state from which they fall to a state which is at a hyperfine wavelength above the ground state. The microwave signal is tuned to a particular frequency so as to repopulate the ground state. In this manner a controlled amount of the light is propagated through the cell and is detected by means of a photodetector.
In order to precisely control the frequency, the wavelength of the propagated light is periodically dithered, that is, varied by small positive and negative amounts, to achieve a dip, which results in a minimum output signal from the detector. Likewise, the microwave signal is also dithered to obtain a microwave induced dip in the detected optical signal. By comparing output signals during a dither, a control means ensures that the wavelength of the propagated light and microwave frequency are precisely controlled. This operation, as well as further details of an atomic clock is more fully described in U.S. Pat. Nos. 5,606,291 and 5,852,386, both assigned to the assignee of the present invention and both of which are hereby incorporated by reference.
There is a need, both in the military and civilian sectors, for an ultra small, highly accurate and extremely low power atomic clock which is frequency fine tunable to account for slight variations from cell to cell. Current models do not meet all of these requirements.
The improved atomic frequency standard of the present invention includes a cell having an active medium and through which is projected a beam of light. A detector of the projected light provides corresponding detector signals to a microprocessor which generates and provides various control signals.
The arrangement includes a microwave cavity adjacent the cell, and a frequency synthesizer operable to provide a microwave signal to the cavity, as well as to provide an output time standard signal.
The frequency synthesizer includes a reference oscillator which provides a reference signal and a voltage controlled oscillator which provides the microwave signal. A fractional-N frequency synthesizer compares an indication of the reference signal with a fractionally divided indication of the voltage controlled oscillator signal. The fractional-N frequency synthesizer provides an output signal which controls the voltage controlled oscillator and varies its frequency as a result of the comparison. The arrangement precisely locks the frequency of the voltage controlled oscillator with the reference oscillator, the output of which may constitute the clock standard output signal.